The lift shaft from the power station to the surface is almost completely free-draining, and the three adits used for the construction of the pressure shafts and lift shaft have been left unlined to provide additional permanent drainage.

It is proposed to continue observation of the water table after completion of the project by measurement of water levels in the drill holes above the site, and by means of pressure-measuring devices installed in the pressure shafts. In the event that the water table builds up to unacceptable levels, it will be possible to drill drainage holes into the rock mass from the adits.

BEHAVIOR OF JOINTED ROCK AROUND EXCAVATIONS

The possibilities for movement along a single joint plane separating two discrete joint blocks in the surface layer of jointed rock surrounding an excavation are first considered, neglecting any interlocking effect of adjacent blocks. This layer has a free surface and is considered to be in a state of two-dimensional stress. The resultant forces parallel to the surface across the joint are represented by P (Fig. 10).

Figure 10. - Condition for Movement Along a Closed Joint

If the joint surface is perfectly plane, sliding into the excavation is prevented by friction only (Fig. 10a). If the joint makes an angle ∝ with the normal to the surface of the excavation equal to or less than Φ, the angle of friction between two smooth plane rock surfaces, then the block will not slide for any value of P. If ∝ is greater than Φ, then the rock will slide. Since Φ is not greater than 45° and is often less, sliding should occur on all joints where ∝ is greater than 45°. In practice it is frequently observed that sliding does not occur under these conditions; thus the sliding must be resisted by forces in addition to friction.

Most joints are either rough or have irregularities, and the irregularities on one joint block very commonly interlock almost perfectly with the complementary irregularities in the adjacent joint block. Even polished slickensided joints generally are smooth only in the direction of the slickensides, but all other directions are irregular. Under these conditions of interlocking, in order for the block to slide, the force acting in the surface of the excavation must be great enough to overcome not only the friction of the joint surface but also to shear off the interlocked irregularities (Fig 10b) These conditions apply to many joints.

If the joint blocks which are separated by a rough joint can move apart or rotate, the amount of interlocked rock is reduced; thus, less force is required to cause failure of the interlocked rough joints and to permit movement along the joint. Considering a group of joint blocks, movement may also be prevented or limited by the interlocking effect of adjacent blocks. For failure to occur it may be necessary for interlocked parts of blocks to be sheared or crushed.

From photoelastic studies and actual measurements it is clear that very high compressive stresses can exist, at least initially, in the surface layer of joint blocks around an opening,